1. Field of the Invention
This invention relates to a method for protecting the materials of construction of a wet oxidation system treating caustic wastewaters, particularly caustic sulfide wastewaters.
2. Information Disclosure Statement
A variety of caustic wastewaters are generated which require treatment before the wastewater is released to the environment. These caustic wastewaters are generated in the petrochemical industry, petroleum refining, pulp and paper manufacture and various chemical manufacturing processes. The caustic solutions are commonly used to remove acidic components such as hydrogen sulfide, H.sub.2 S, mercaptans, RSH, phenols, ArOH, and organic acids, RCO.sub.2 H, from gas and liquid streams.
The contaminated caustic wastewaters represent a formidable disposal problem due to their caustic content as well as the acidic components therein. Neutralization of the caustic wastewaters by acid addition can result in release of the acidic components. Therefore it is essential to convert the acidic components to a form suitable for release to the environment. Further, there may be additional components present in the caustic wastewater which adds to the Chemical Oxygen Demand (COD) of the wastewater. These components include various carbonaceous materials including oils and polymers.
Wet oxidation is the preferred method of treatment for caustic wastewaters since the products of oxidation are inorganic sulfate, carbon dioxide and water. Also, the oxidation is carried out within a closed system which prevents transfer of pollutants to the atmosphere. The highly alkaline nature of these caustic wastewaters requires special materials of construction for wet oxidation systems employed in their treatment. The nickel-based alloys, such as Inconel 600, are well suited to withstand the elevated temperatures and pressures employed in the wet oxidation process for caustic wastewater treatment. The ferrous-based alloys, such as carbon steel, stainless steel or superstainless steel, may be considered for lower temperature wet oxidation treatment of caustic wastewaters. However, under wet oxidation conditions, corrosion is a serious problem for the ferrous-based alloys.
In U.S. Pat. No. 3,761,409 McCoy et al. disclose a continuous process for the air oxidation of sulfidic, ammoniacal sour water where feed water is adjusted to a pH between about 6 to 13 and the oxidation occurs at 250.degree. F. to 520.degree. F. at 75 to 800 psig with up to 500% excess oxygen based on the stoichiometric conversion of sulfide to sulfate.
Chowdhury in U.S. Pat. No. 4,350,599 discloses wet oxidation of caustic liquor where carbon dioxide generated by the oxidation is used to reduce the pH of the caustic feed liquor to below 11. Maintaining the feed below pH 11.0 but above 7.0 prevents corrosion of the less expensive stainless steel wet oxidation system.
As mentioned above, the nickel-based alloys are resistant to corrosion by caustic sulfide wastewaters under wet oxidation conditions, provided the pH of the wastewater is maintained on the alkaline side, that is above pH 7. Likewise, the ferrous-based alloys provide their best corrosion resistance at neutral or alkaline pH, provided the temperature of the system remains relatively low. The wet oxidation of sulfide wastewaters generates acidic species which consume alkalinity. Depending on the components present, their concentration, and the pH of the caustic sulfide wastewater, wet oxidation may produce an oxidized wastewater in which the pH is acidic, i.e. all alkalinity is consumed, and which is highly corrosive to the nickel-based or ferrous-based wet oxidation system.
Beula et al. in U.S. Pat. No. 5,082,571 have devised a process which relates the species present in the caustic liquor to the amount of caustic required to maintain an excess of alkalinity in the liquor during wet oxidation treatment. This process allows a nickel-based alloy wet oxidation system to safely treat caustic sulfide liquor without excessive corrosion to the materials of construction of the system. The process requires extensive analysis of the raw feed liquor and gives best results with a constant composition feed. Problems can result where feed composition changes and alkalinity consuming species increase, causing a drop in the pH of the oxidized wastewater.
To overcome this problem, I have devised a method of determining a drop in pH for a caustic wastewater undergoing wet oxidation treatment which allows for the adjustment of the raw feed composition, i.e. pH, before corrosion to the materials of construction of the wet oxidation system occurs. It must be recognized that the corrosion problems need careful consideration in that the integrity of the pressurized wet oxidation system is important for both safety and economic reasons.